This invention relates to the field of solventless silicone resins. A solventless silicone resin is one that can be either applied to a surface or used as an impregnating or encapsulating resin without the necessity of diluting it with a solvent. Of the solventless resins the most commercially acceptable kind are those that are cured by the addition of SiH to Si vinyl. Such resins are useful in two major applications; namely, coating and encapsulation.
The properties of the resin needed to make a successful coating resin are different from those needed to make a successful impregnant or encapsulant. This is particularly true with regard to a phenomenon known as thermal shock. Thermal shock refers to the ability of the resin to withstand rapid changes in temperature over wide ranges. When a resin is used as a coating, the only contact between the resin and the substrate usually metal, is at the surface of the two, hence, a difference in expansion and contraction of the resin and the substrate places stress only in one area of the resin. Furthermore, a coating resin is always in a thin film and as is well known, thin films are less subject to thermal shock than deep sections. On the other hand, an encapsulating or impregnating resin used in the electronic industries, for example, has component parts embedded within the resin matrix which parts may have widely different coefficients of expansions from that of the resin. Under such conditions fluctuations in temperature place an infinitely greater stress on the resin than is placed on a same resin in a coating use. For this reason, resins which are eminently satisfactory for coating applications, such as for use on traction motors, are completely inadequate as potting or encapsulating compounds for electrical or electronic equipment.
It is the object of this invention to produce a solventless silicone resin having improved thermal shock in both the filled and unfilled state which render it eminently useful as an encapsulating resin.
One of the things which must be considered in formulation of a solventless-type resin is not only the composition of the base resin, but also the composition of the crosslinker. For example, the same base resin may give entirely different thermal shock properties depending upon the composition of the crosslinker. The success of the resins of this invention is due to the crosslinkers used, particularly with respect to the dimethyl content.